Possible to redirect from apache to groovebasin?

I would like to run groovebasin on my Freedombox, where I use pagekite to forward the https port from apache to a publicly available IP address. For this to work with groovebasin, I would like to set up groovebasin behind apache. But I am unable to get this to work. I've tried using this content in /etc/apache2/conf-available/groovebasin.conf and running a2enconf groovebasin:

```
##
## On all sites, provide Groovebasin on a path: /groovebasin
## Allow all valid users.
##

ProxyPass http://localhost:16242

AuthType basic
AuthName "FreedomBox Login"
AuthBasicProvider ldap
AuthLDAPUrl "ldap:///ou=users,dc=thisbox?uid"
AuthLDAPGroupAttribute memberUid
AuthLDAPGroupAttributeIsDN off
Require valid-user

```

The problem is that groovebasin do not understand that all its URLs have changed, and ask the browser to load /app.css, /app.js and stuff under /vendor/. Is there a way to tell groovebasin what URL base to use when sending URLs to the browser?